MEMS chip and package method thereof

ABSTRACT

The present invention proposes a MEMS chip and a package method thereof. The package method comprises: making a capping wafer by: providing a first substrate and forming an etch stop layer on the first substrate; making a device wafer by: providing a second substrate and forming a MEMS device and a material layer surrounding the MEMS device on the second substrate; bonding the capping wafer and the device wafer; after bonding, etching the first substrate to form at least one via; etching the etch stop layer through the via; etch the material layer; and forming a sealing layer on the first substrate.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a Micro-Electro-Mechanical System(MEMS) chip and a package method thereof.

2. Description of Related Art

In a process for making a MEMS chip, an internal MEMS device, such as amicro-acoustical sensor, gyro-sensor, or accelerometer, is oftenrequired to be packaged in a sealed space to maintain its stability.Conventionally, a package process is performed after the MEMS device is“released”, i.e., after the MEMS device becomes a movable structure byan etching step. A device wafer with the MEMS device formed therein isbonded with a capping wafer by a bonding material such as aluminum orglass material. However, when the MEMS device is made by a CMOScompatible process, because the MEMS device is packaged after it isreleased, and since the MEMS device made by the CMOS compatible processuses aluminum or other material which can not sustain high temperature,the package method suffers many constraints. In addition, the MEMSdevice made by the CMOS compatible process is also more likely to bedamaged during the package process.

The present invention provides a MEMS chip structure and a packagemethod thereof to reduce the impact by the temperature and enhance theyield of the MEMS device.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a packagemethod of a MEMS chip, which solves the foregoing problem by packagingthe MEMS chip before releasing the MEMS device.

Another objective of the present invention is to provide a MEMS chipwith the foregoing package method.

In order to achieve the foregoing objectives, in one perspective of thepresent invention, it provides a package method of a MEMS chip,comprising the following steps:

-   -   making a capping wafer by: providing a first substrate; and        forming an etch stop layer on the first substrate;    -   making a device wafer by: providing a second substrate; and        forming a MEMS device and a material layer surrounding the MEMS        device on the second substrate;    -   bonding the capping wafer and the device wafer;    -   after bonding, etching the first substrate to form at least one        via;    -   etching the etch stop layer through the via; and    -   etching the material layer.

After the foregoing steps are completed, preferably, a sealing layer isdeposited on the first substrate.

In the foregoing method, preferably, the etch stop layer and thematerial layer are made of the same material, or have a similar etchrate to a certain etchant. The etch stop layer is only required to coverthe via formed in the first substrate; its pattern is not required to bevery precise.

The capping wafer and the device wafer can be bonded by hermeticalpackage or non-hermetical package. The hermetical package for exampleincludes glass frit or solder, wherein the solder material for examplecan be a material including metal or alloy. An insulation layer ispreferably formed on the device wafer when metal or alloy is employedfor hermetical package. In non-hermetical package, for example,photosensitive polymer can be used as a bonding material. The pattern ofthe bonding material can be defined by lithography or by lithography andetching, depending on the property of the bonding material.

In another perspective of the present invention, it provides a MEMS chipcomprising:

-   -   a first segment including: a sealing layer; and a first        substrate below the sealing layer wherein the substrate has a        cavity being sealed;    -   a second segment including: a second substrate; and a MEMS        device on the second substrate; and    -   a bonding layer bonding the first segment and the second        segment.

The objectives, technical details, features, and effects of the presentinvention will be better understood with regard to the detaileddescription of the embodiments below, with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 show the first embodiment for making a capping wafer accordingto the present invention.

FIGS. 4-6 show the second embodiment for making the capping waferaccording to the present invention.

FIGS. 7-8 show the first embodiment for making a device wafer accordingto the present invention.

FIGS. 9-13 are steps of bonding the capping wafer and the device waferaccording to the present invention.

FIGS. 14-18 show the second embodiment for making the device wafer, andthe steps of bonding the capping wafer and the device wafer according tothe present invention.

FIGS. 19-21 show the third embodiment for making the capping waferaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings as referred to throughout the description of the presentinvention are for illustration only, to show the interrelationshipsbetween the process steps and between the layers, but not drawnaccording to actual scale.

One characteristic of the present invention is that a wafer including aMEMS device (hereinafter referred to as “device wafer”) is first bondedto a capping wafer, and after bonding and packaging, the MEMS deviceinside the package is etched to release. Because the MEMS device has notyet been released during the package process (the MEMS device is still apart of the device wafer and not movable), it is more capable ofsustaining high temperature and less likely to be damaged.

According to the present invention, there are various methods to makeand bond the capping wafer and the device wafer. FIGS. 1-3 show thefirst embodiment of the present invention for making the capping wafer.As shown in FIG. 1, a first substrate 11, e.g., a silicon substrate, isprovided, and next an etch stop layer 12 is deposited on the substrate11. The material selection of the etch stop layer 12 will be discussedlater. Because the etch stop layer 12 is provided for masking thesubstrate 11 during etching, when the substrate 11 is made of silicon,the etch stop layer 12 should have a higher etch selectivity to silicon;when the substrate 11 is made of another material, the etch stop layer12 should have a higher etch selectivity to said other material. Next,the etch stop layer 12 is patterned, for example by inductive couplingplasma (ICP) etch. Afterward, a first bonding layer 13 is formed on thecapping wafer, and patterned. In one embodiment, the first bonding layeris selected from a material which can be patterned by photolithography,such as one of the followings: photosensitive polymer, parlene, PDMS(Polydimethylsiloxane), and photo-imagable resin. In this embodiment, itis only required to perform a photolithography step on the first bondinglayer according to a desired pattern, and then removing the undesiredpart. In another embodiment, the first bonding layer 13 is made of glassfrit or solder. In this embodiment, the pattern needs to be defined byanother way, such as by lithography and etch. When the bonding layer 13is made of solder, the material selection will be discussed later.

When the first bonding layer 13 is not selected from a material whichcan be patterned by photolithography, it is not necessary to form theetch stop layer 12 and the bonding layer 13 in order. Instead, they canbe formed in either the order as shown in FIGS. 1-3 or the order asshown in FIGS. 4-6, wherein the first bonding layer 13 is formed firstand the etch stop layer 12 next.

A method for making the device wafer is shown in FIGS. 7-8. First, asecond substrate 21, for example made of silicon, is provided. A MEMSdevice 24 and a material layer 22 surrounding the MEMS device are formedon the second substrate 21. The MEMS device 24 can be of any shape andany number of layers; what is shown is only for example. The materiallayer 22 can be made of any material used for insulation between metallayers in a standard CMOS process, and it may be of the same material asthat of the etch stop layer 12 or not, but preferably the same. In onepreferred embodiment, the etch stop layer 12 and the material layer 22are both made of oxide, such as silicon dioxide. At or after forming theMEMS device 24 and the material layer 22, a bond pad 26 is formed aswell, for connection to external circuitry. After the abovementionedsteps are completed, a second bonding layer 23 corresponding to thefirst bonding layer 13 is formed subsequently; the material selectionthereof will be discussed later.

Referring to FIGS. 9-13, after the capping wafer and the device waferare completed (which may be done in parallel), they are bonded by meansof the first bonding layer 13 and the second bonding layer 23. There arevarious combinations of the materials of the first bonding layer 13 andthe second bonding layer 23. For example, the first bonding layer 13 canbe made of the abovementioned photosensitive polymer, including:parlene, PDMS, or photo-imagable resign; and the second bonding layer 23can be made of a material which is the same as the first bonding layer13, or epoxy. In this case, the capping wafer and the device wafer arebonded by non-hermetical package. Or, the first bonding layer 13 and thesecond bonding layer 23 can be bonded by glass frit; in this case ahermetical package is formed.

After the capping wafer and the device wafer are bonded, in oneembodiment, it is preferred to reduce the thickness of the firstsubstrate 11, the second substrate 21, or both, by grinding; forexample, the first substrate 11 may be controlled within a thicknessrange between 100 μm˜200 μm. Next, the first substrate 11 is etched, forexample by ICP etch, such that at least one via is formed as shown inFIG. 11. The effect of the etch stop layer 12 can be understood by now,and because the pattern of the etch stop layer 12 is only required tocover the via, it does not have to be very precise. Next, through thevia formed in the first substrate 11, the etch stop layer 12 and thematerial layer 22 are removed by etching as shown in FIG. 12. Thisexplains why the etch stop layer 12 and the material layer 22 arepreferably made of the same material. Yet, they can certainly be made ofdifferent materials as well; in this case, one may need to use differentetchants to etch the etch stop layer 12 and the material layer 22. Ifthe etch stop layer 12 and the material layer 22 are both made of oxide,the etchant for example can be vapor hydrogen fluoride (HF). After thematerial layer 22 is etched, the MEMS device 24 is released and becomesa movable part. At last, preferably, a sealing layer 31 is provided toseal the via in the first substrate 11; the sealing layer 31 can be madeof any material capable of achieving the sealing function, including butnot limited to metal, as shown in FIG. 13. The wafer as shown in FIG. 13may subsequently be sliced into multiple chips (one wafer containsmultiple chips and FIGS. 1-21 of the present invention show only onechip for illustration).

FIGS. 14-18 illustrate another embodiment of the present invention. Inthis embodiment, a capping wafer and a device wafer are bonded bysoldering; the bonding layer 23 is made of a material including metal oralloy. The first bonding layer 13 and the second bonding layer 23 aremade of a material suitable for soldering, including but not limited to:metal, aluminum-silicon alloy, silicon-gold alloy, tin-silver alloy,gold-germanium alloy, gold-tin alloy, or gold-lead alloy. In such caseas shown in FIG. 15, after a bond pad 26 is provided on the secondsubstrate 21, preferably, an insulation layer 25 is deposited on thebond pad 26 to prevent the bond pad 26 from being short-circuited to thesecond bonding layer 23. The material of the insulation layer 25 ispreferably different from the etch stop layer 12 and the material layer22; the insulation layer 25 for example can be made of a materialincluding silicon carbide (SiC) or amorphous silicon. Subsequent stepsafter FIG. 15 are shown in FIGS. 16-18, wherein the capping wafer andthe device wafer are soldered; the substrate is grinded; the firstsubstrate 11 is etched to formed a via; the etch stop layer 12 andmaterial layer 22 are etched and removed; a sealing layer 31 is coated,etc. These steps are similar to those in the previous embodiment andtheir details are thus omitted here.

FIGS. 19-21 illustrate another embodiment of the present invention. Inthis embodiment, the method for making the capping wafer is slightlydifferent from the foregoing embodiment, as the following: afterproviding a first substrate 11, as shown in FIG. 19, the first substrate11 is etched according to a pattern of an etch stop layer 12 to beformed on it, the pattern being such that it covers at least theposition of a via to be formed in the first substrate 11 in a subsequentprocess. Subsequently, an etch stop layer 12 is deposited in the regionas shown in FIG. 20, and next a first bonding layer 13 is further formedas shown in FIG. 21. The capping wafer formed by this embodiment can bebonded to the device wafer by any of the methods described previously,and the details of the bonding process are omitted here.

The present invention has been described in considerable detail withreference to certain preferred embodiments thereof. It should beunderstood that the description is for illustrative purpose, not forlimiting the scope of the present invention. Those skilled in this artcan readily conceive variations and modifications within the spirit ofthe present invention. In view of the foregoing, the spirit of thepresent invention should cover all such and other modifications andvariations, which should be interpreted to fall within the scope of thefollowing claims and their equivalents.

1-21. (canceled)
 22. A MEMS chip comprising: a first segment including:a sealing layer; and a first substrate below the sealing layer whereinthe substrate has a cavity being sealed; a second segment including: asecond substrate; and a MEMS device on the second substrate; and abonding layer bonding the first segment and the second segment.
 23. TheMEMS chip of claim 22, wherein the second segment further includes abond pad on the surface of the second substrate.
 24. The MEMS chip ofclaim 23, wherein the second segment further includes an insulationlayer on the bond pad.
 25. The MEMS chip of claim 24, wherein theinsulation layer includes silicon carbide (SiC) or amorphous silicon.26. The MEMS chip of claim 22, wherein the bonding layer is made of amaterial including: photosensitive polymer, glass frit or solder. 27.The MEMS chip of claim 26, wherein the photosensitive polymer includes:parylene, PDMS, epoxy, or photo-imagable resin.
 28. The MEMS chip ofclaim 26, wherein the solder is made of a material including: metal orone of the following alloys: aluminum-silicon alloy, silicon-gold alloy,tin-silver alloy, gold-germanium alloy, gold-tin alloy, and lead-tinalloy.